1. Field of the Invention
The present invention relates to a laminated piezoelectric device. More specifically, it relates to a laminated piezoelectric device which enables both prevention of migration and suppression of characteristic deterioration caused by pyroelectric effect.
2. Description of the Related Art
A piezoelectric device is a device mutually converting mechanical displacement and electric displacement by utilizing piezoelectric effect and inverse piezoelectric effect. Such piezoelectric device can be obtained by forming and firing piezoelectric ceramics to make a device body, and then forming electrodes thereon and applying direct electric fields to uniform a direction of spontaneous polarization.
The mechanical displacement obtained by the piezoelectric device is minute. Therefore, the piezoelectric device is used for various products as actuators for the purpose that accurate and precise controls are required, for instance. Specifically, it is used for lens drive, HDD head drive, ink-jet printer head drive, fuel injection valve drive and so on.
Further, when laminating piezoelectric layers to make a laminated piezoelectric device so that a larger displacement can be obtained, edge portions of internal electrodes sandwiching piezoelectric layers are exposed on the surface of the device body. In this case, depending on moisture in the air and the like, migration might cause between internal electrodes having different polarities.
Further, for instance, if such piezoelectric device is applied to portable electronic devices and the like, it will be exposed to temperature changes due to surrounding environmental changes.
If the piezoelectric device is exposed to such temperature changes, polarization caused by pyroelectric effect becomes a problem. Particularly, in the process of lowering of temperature, polarizability decreases since a polarization occurs, opposite to the spontaneous polarization, in the piezoelectric device due to pyroelectric effect. This decreased polarizability does not recover even if the temperature rises again. Therefore, there is a problem that the characteristics deteriorate since the polarizability gradually decreases and a desired displacement cannot be obtained, when the process of lowering of temperature is repeated.
In order to deal with the above problems, the following Reference 1 describes about “a piezoelectric device in which internal electrodes, which are exposed on the sides of laminated piezoelectric body, are covered with exterior materials that prevents migration, and conductive particles are dispersed in the exterior materials”.
Furthermore, the following Reference 1 describes that “with this piezoelectric device, it enables to prevent migration by the exterior materials that prevents migration, and also to suppress the decrease of polarizability caused by pyroelectric effect”.
[Reference 1] International Publication No. 2007/052599
However, for making a resistance value of the exterior material in the range that could prevent migration and suppress the decrease of polarizability, there is a problem that it is difficult to adjust the resistance value since fluctuation of resistance value of resin composing the exterior materials and the content of conductive particles and the like should be taken into consideration.
When the conductive particles aggregate in the exterior materials, there were problems that the resistance value of the aggregated portion locally decreases and then a short circuit might be caused through the portion. Particularly, this problem becomes prominent as the distance (thickness of piezoelectric layer) between internal electrodes gets smaller.
Further, there is a possibility that it might be unable to adequately prevent moisture permeation if providing several functions to the exterior materials. Furthermore, when a substrate of the piezoelectric devices is cut after firing to obtain individual piezoelectric devices, there were problems that fluctuation of resistance value might be generated since the broken internal electrodes extend along a penetration direction of cutting blades.